1. Field of the Invention
The present invention relates to a roller transfer device for use in an electrophotographic apparatus of, e.g., a facsimile, a laser printer, and a copying machine.
2. Description of the Related Art
FIG. 1 shows an example of an electrophotographic apparatus used as a recording section of a facsimile or a printer.
In FIG. 1, reference numeral 1 denotes a photoreceptor drum manufactured by coating a photoconductive material on the outer circumferential surface of a cylindrical aluminum tube element. A charger 2, an exposing device 3, a developing unit 4, a roller transfer device 5, and a cleaning unit 6 are arranged along the outer circumferential surface of the photoreceptor drum 1.
This electrophotographic apparatus performs image formation by the action explained below.
The charger 2 charges the photoconductive layer of the photoreceptor drum 1 to, e.g., -500 V, and the exposing device 3 exposes the photoconductive layer of the photoreceptor drum 1 in accordance with an image to be recorded. Upon exposure, the charge in the exposed portion of the photoconductive layer of the photoreceptor drum 1 is removed, and an electrostatic latent image is formed on that portion.
In the developing unit 4, a developing bias which is a low voltage such as -200 V having the same polarity as the charging potential of the photoconductive layer of the photoreceptor drum 1 is applied to a development roller. Consequently, no toner sticks to the charged portion of the photoreceptor drum 1 since the potential of the photoreceptor drum 1 is higher in that portion. On the other hand, the toner adheres to the exposed, charge-removed portion of the photoreceptor drum 1 because the potential of the photoreceptor drum 1 is lower in that portion.
Subsequently, sheets of recording paper P stored in a paper feed cassette (not shown) are picked up by a paper feed roller and separately fed one by one to the photoreceptor drum 1 by a separation roller.
The roller transfer device 5 includes a transfer roller 7 which is rotated in contact with the photoreceptor drum 1. To form a stable contact portion (nip), additional rollers (not shown) having a diameter smaller than outside diameter of the roller can be provided at the two ends of the core shaft of the transfer roller 7. A core shaft 8 of the transfer roller 7 is connected to a transfer power supply 15 and applied with a transfer bias.
While the recording paper P is being fed between the transfer roller 7 and the photoreceptor drum 1, charge (e.g., +1.35 kV) having opposite polarity to that of the toner is applied to the transfer roller 7, thereby transferring the toner sticking to the photoconductive layer of the photoreceptor drum 1 onto the recording paper P.
After the transfer, the residual toner that has not been transferred from the photoconductive layer of the photoreceptor drum 1 is removed by the cleaning unit 6.
FIG. 2 illustrates the structure of the transfer roller 7 used in the conventional roller transfer devices. Referring to FIG. 2, the outer circumferential surface of the core shaft 8 made of a metal is covered with a conductive urethane sponge elastic layer 9. A conductive vinyl chloride film 10 is formed as an adhesive layer on the outer circumferential surface of the conductive urethane sponge elastic layer 9. A fluorine-based seamless film 11 is formed as a resistance layer on the outer circumferential surface of the vinyl chloride film 10. Lastly, the transfer roller 7 with this structure is heated to bond these layers. Since it is difficult to directly fuse the fluorine-based seamless film resistance layer 11 and the conductive urethane sponge elastic layer 9 due to good release characteristics of the fluorine-based seamless resistance layer 11, these layers 9 and 11 are bonded by bonding their opposing surfaces via the vinyl chloride film 10.
The function of the conductive urethane sponge elastic layer 9 is to make the transfer roller 7 elastically contact with the outer circumferential surface of the photoreceptor drum 1. The fluorine-based seamless film 11 allows the surface of the transfer roller 7 to reliably feed recording paper, and suppresses wear of the transfer roller 7. Also, the layers 9, 10, and 11 are given conductivity in order to enable voltage application to the transfer roller 7.
In the above structure, the higher the adhesion temperature the higher the adhesive strength with which the fluorine-based seamless film resistance layer 11, the adhesive layer 10, and the conductive urethane sponge elastic layer 9 are bonded. The fluorine-based seamless film resistance layer 11 shows no large deformation even when heated to temperatures near the softening point only for a short time. On the other hand, the conductive urethane sponge elastic layer 9 is a very soft resin compared to the fluorine-based seamless film resistance layer 11. Accordingly, when heated to the softening point or higher temperatures only for a short time the conductive urethane sponge elastic layer 9 tends to deteriorate, e.g., to bring about an unwanted deformation or to impair the desired elasticity. This deterioration decreases the transfer roller performance. From this viewpoint, it is necessary to set the adhesion temperature to a temperature lower than the heat resistant temperature of the conductive urethane sponge. Since the conductive urethane sponge 9 deforms at about 150.degree. C., the adhesion temperature of the fluorine-based seamless film resistance layer 11 and the conductive urethane sponge layer 9 is set at below about 150.degree. C. Unfortunately, the fluorine-based seamless film resistance layer 11 has very good release characteristics and hence is difficult to bond, in comparison with the conductive urethane sponge elastic layer 9. Accordingly, the conventional transfer rollers have the problem that no satisfactory adhesive strength can be attained between the fluorine-based seamless film resistance layer 11 and the adhesive layer 10 and these layers readily peel, even when heating is done at below a temperature of approximately 150.degree. C.